The present invention relates to a power amplifier which is suitable for use in nonlinear amplification in high-frequency bands.
Conventionally known as this kind of power amplifier is a class "F" amplifier (Tayler, V. J., "A New High Efficiency High Power Amplifier", Marconi Review, vol. 21, No. 130, pp. 96-109, 3rd Quarter, 1958, for example). The class "F" amplifier has such an arrangement as shown in FIG. 1, in which an input signal from an input terminal 11 is applied to the gate of an FET 12 serving as an amplifying element and the FET 12 has its source grounded and its drain connected to a power source terminal 14 via a choke coil 13 which blocks high frequencies. The drain of the FET 12 is grounded via a filter 15 which permits the passage therethrough of a second harmonic component of the input signal and at the same time it is connected to an output terminal 18 via a filter 16 which permits the passage therethrough of the fundamental wave component of the input signal and via an impedance matching circuit 17.
The FET 12 is supplied with a class "A" or "AB" bias and generates harmonics of the input signal fed from the input terminal 11. The filter 15 acts as a short-circuit load for a second harmonic component and as an open-circuit load for a third harmonic (an odd order harmonic) component. As a result of this, a current i(t), which is a superimposition of the fundamental and second harmonic components of the input signal, flows as indicated by the solid line in FIG. 2A, and a voltage v(t), which is a superimposition of the fundamental and third harmonic components of the input signal, is created 180 degrees out of phase with the current i(t) as indicated by the broken line in FIG. 2A. The amplifying operation of the FET 12 is close to a switching operation. An overlap of the waveforms of the current i(t) and the voltage v(t) in FIG. 2A is a loss which is dissipated by the FET 12. In this instance, since the current i(t) and the voltage v(t) are 180 degrees out of phase with each other as mentioned above, the loss is small, and accordingly a high efficiency can be achieved in the amplifying operation. It is known in the art that the closer to the half wave of a sine wave the waveform of the current i(t) is and the closer to a square wave the waveform of the voltage v(t) is while the less these waveforms overlap, the more the efficiency is increased.
However, since the class "F" amplifier has at its output a short-circuit load for the second harmonic component, an increase in the gain of the FET 12 will cause the second harmonic components of internal electronic disturbance in the gate of the FET 12 to produce a large short-circuit current through the filter 15. In practice, the filter 15 has a certain passing band width, and hence will allow a larger current flow therethrough caused not only by the second harmonic component but also by an electronic disturbance (noise) of frequencies close to that of the former; therefore, the class "F" amplifier lacks stability in operation.
Where the class "F" amplifier is employed in a high-frequency band such as the microwave band, the filters 15 and 16 are each formed by a strip line as shown in FIG. 3, and the length of the filter 15 is specifically selected to be a quarter of the wavelength of the fundamental wave of the input signal. The filter 15 is grounded via a DC blocking condenser 21 and the filter 15 is connected to the output terminal 18 via a DC blocking condenser 22. Accordingly, the filter 15 functions, when seen from the drain terminal of the FET 12, as an open-circuit load for the fundamental wave and as a short-circuit load for the second harmonic. In practice, however, it has been difficult, even through utilization of such strip line filters, to obtain a class "F" amplifier suitable for operation at frequencies above 1 GHz or so, because the lengths of the strip lines must become unpractically short. Moreover, since the filter 15 has to be grounded at one end through a through-hole to a grounding conductor formed on the opposite surface of a circuit board, complicated and high precision manufacturing techniques are required for obtaining the intended filter characteristic. Also for this reason, the class "F" amplifier is not suitable for use especially in high-frequency bands.